U.S. patent number 4,533,795 [Application Number 06/511,640] was granted by the patent office on 1985-08-06 for integrated electroacoustic transducer.
This patent grant is currently assigned to American Telephone and Telegraph, AT&T Bell Laboratories. Invention is credited to John C. Baumhauer, Jr., Harold J. Hershey, Tommy L. Poteat.
United States Patent |
4,533,795 |
Baumhauer, Jr. , et
al. |
August 6, 1985 |
Integrated electroacoustic transducer
Abstract
An electroacoustic transducer, primarily in the form of a
capacitive microphone, for incorporation into a semiconductor
substrate. The vibrating element comprises a largely nontensioned
diaphragm, such as an epitaxial layer formed on the semiconductor
substrate, so as to greatly reduce its mechanical stiffness. The
substrate is etched away in the desired area to define the
diaphragm and form an acoustic cavity. A continuous array of
microscopic holes is formed in the backplate to cut down the
lateral flow of air in the gap between capacitor electrodes. Narrow
gaps made possible by the hole array allow low voltage diaphragm
biasing. In at least one embodiment, the acoustic input can be
provided through the air hole array. An acoustic port may be added
to alter the frequency response of the device, and a back closure
provided to act as a rear acoustic cavity and an EMI shield.
Inventors: |
Baumhauer, Jr.; John C.
(Indianapolis, IN), Hershey; Harold J. (Indianapolis,
IN), Poteat; Tommy L. (Bridgewater, NJ) |
Assignee: |
American Telephone and
Telegraph (New York, NY)
AT&T Bell Laboratories (Murray Hill, NJ)
|
Family
ID: |
24035781 |
Appl.
No.: |
06/511,640 |
Filed: |
July 7, 1983 |
Current U.S.
Class: |
381/174;
29/25.35 |
Current CPC
Class: |
G01L
9/0073 (20130101); H04R 1/06 (20130101); H04R
19/005 (20130101); Y10T 29/42 (20150115); H04R
1/225 (20130101) |
Current International
Class: |
G01L
9/00 (20060101); H04R 1/22 (20060101); H04R
19/00 (20060101); H04R 007/18 () |
Field of
Search: |
;179/111E,111R
;29/594,592E,25.35 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
84608 |
|
Oct 1982 |
|
EP |
|
2094098 |
|
Sep 1982 |
|
GB |
|
Other References
"On the Acoustical Resistance Due to Viscous Losses in the Air Gap
of Electrostatic Transducers", Z. Skvor, Acustica, vol. 19, pp.
295-299, (1967/1968). .
"Optimization of a Ridge Backplate for Electret Microphones", H. S.
Madsen, Journal of the Acoustical Society of America, vol. 53, No.
6, pp. 1616-1619, (1973)..
|
Primary Examiner: Rubinson; Gene Z.
Assistant Examiner: Schroeder; L. C.
Attorney, Agent or Firm: Birnbaum; Lester H.
Claims
What is claimed is:
1. An electroacoustic transducer formed within a semiconductor
substrate comprising
an essentially nontensioned diaphragm comprising a semiconductor
material having a mechanical bending moment stiffness such that it
vibrates in response to an input signal at audio and/or ultrasonic
frequencies; and
a pair of electrodes placed with respect to said diaphragm so that
the electric field between the electrodes varies in relationship
with the vibrating diaphragm to permit conversion between
electrical and acoustic signals.
2. The device according to claim 1 wherein the transducer is a
microphone and one of the electrodes is formed to vibrate with the
diaphragm such that the capacitance varies in response to an
acoustic signal incident on said diaphragm.
3. The device according to claim 2 wherein the diaphragm is a
single crystalline epitaxial layer formed over the surface of a
semiconductor substrate.
4. The device according to claim 3 wherein the thickness of the
epitaxial layer is in the range 1-5 .mu.m.
5. The device according to claim 2 wherein the area of the
diaphragm is less than 5 mm.sup.2.
6. The device according to claim 1 wherein the effective mechanical
lumped parameter stiffness of the diaphragm, associated with the
diaphragm's linearly averaged deflection, is less than 5500
N/m.
7. The device according to claim 3 wherein the epitaxial layer is
formed over a surface layer in the substrate which has a higher
impurity concentration than the bulk of the substrate.
8. The device according to claim 1 wherein an air gap is formed
between the two electrodes and the device further comprises a
continuous array of small holes extending through one of the
electrodes to the air gap, said array covering essentially the
entire area of that portion of the electrode which is co-extensive
with the diaphragm.
9. The device according to claim 1 wherein the transducer is
adapted for operation with a dc bias across the electrodes of less
than 5 volts.
10. The device according to claim 1 wherein an air gap is formed
between the electrodes and the device further comprises:
a front acoustic cavity defined by a portion of the semiconductor
substrate adjacent to the diaphragm;
a rear acoustic cavity adjacent to one of the electrodes and
defined by a member which also shields the transducer from
electromagnetic interference;
acoustic venting means comprising an array of holes connecting the
air gap to the back acoustic cavity; and
an acoustic port defined by an element adjacent to the front
acoustic cavity, the size of the port being such as to produce a
desired frequency characteristic for the transducer.
11. An electroacoustic transducer formed within a semiconductor
substrate comprising:
a diaphragm which vibrates in response to an input signal at audio
and/or ultrasonic frequencies;
a pair of electrodes placed with respect to said diaphragm so that
the electric field between the electrodes varies in relation to the
vibrating diaphragm to permit conversion between electrical and
acoustic signals, the electrodes defining an air gap therebetween;
and
acoustic venting means comprising a continuous array of small holes
extending through one of the electrodes to the air gap, said array
covering essentially the entire area of that electrode which is
co-extensive with the diaphragm, where the density of the holes in
the array is at least 50 per mm.sup.2 and the diameter of the holes
is less than 100 .mu.m.
12. The device according to claim 11 wherein the transducer is a
microphone and one of the electrodes is formed to vibrate with the
diaphragm such that the capacitance varies in response to an
acoustic signal incident on said diaphragm.
13. The device according to claim 11 wherein a backplate layer is
included adjacent to said one of the electrodes and said array of
holes is included through the layer.
14. The device according to claim 11 wherein the spacing between
electrodes is less than 3.0 .mu.m when no dc bias is supplied to
the electrodes.
15. The device according to claim 11 wherein the area of the
diaphragm is less than 8 mm.sup.2.
16. The device according to claim 1 wherein the spacing between the
electrodes is uniform when no bias is supplied to the
electrodes.
17. The device according to claim 11 wherein the transducer is
adapted to operate with a dc bias across the electrodes of less
than 5 volts.
18. The device according to claim 11 wherein the dynamic
magnification factor of the output response of the transducer is
greater than or equal to -3.0 dB.
19. The device according to claim 11 where the radius, r,
center-to-center spacing, 2.xi., and number, N, of holes in the
array satisfies the relationships: ##EQU8## where D is the minimum
desirable dynamic magnification factor, .omega. is the highest
radian frequency of the input signal to be transmitted,
.omega..sub.n is the natural radian frequency of vibration of the
diaphragm, R.sub.a is the damping level of the air in the gap,
R.sub.c is the critical damping level, X is the local
radius-of-action associated with each hole, h.sub.e is the average
distance between the electrodes when the desired dc bias is
supplied thereto, .lambda. is a factor relating X and .xi. for the
geometry of the hole array, A is the area of the diaphragm, and
.eta. is the coefficient of dynamic viscosity of air.
20. The device according to claim 11 further comprising:
a front acoustic cavity defined by a portion of a semiconductor
substrate adjacent to the diaphragm;
a back acoustic cavity adjacent to the air hole array and defined
by a member which also shields the transducer from electromagnetic
interference; and
an acoustic port defined by an element adjacent to the front
acoustic cavity, the size of the port being such as to produce a
desired frequency response characteristic for the transducer.
21. The device according to claim 12 wherein the microphone is
adapted to receive the acoustic signal through the hole array.
22. An electroacoustic transducer formed within a semiconductor
substrate comprising:
a diaphragm which vibrates in response to an input signal at audio
and/or ultrasonic frequencies;
a front acoustic cavity defined by a portion of the semiconductor
substrate adjacent to the diaphragm;
a pair of electrodes placed with respect to said diaphragm so that
the electric field between the electrodes varies in relation with
the vibrating diaphragm to permit conversion between electrical and
acoustic signals and so that an air gap is formed between the
electrodes;
a back acoustic cavity adjacent to one of the electrodes and
defined by a member which also shields the transducer from
electromagnetic interference;
acoustic venting means comprising an array of holes connecting the
air gap to the back acoustic cavity; and
an acoustic port defined by an element adjacent to the front
acoustic cavity, the size of the port being such as to produce a
desired frequency response characteristic for the transducer.
23. The device according according to claim 22 wherein the
transducer is a microphone and one of the electrodes is formed to
vibrate with the diaphragm such that the capacitance varies in
response to an acoustic signal incident on said diaphragm.
24. The device according to claim 22 wherein the acoustic port is
defined in a carrier to which the semiconductor substrate is
attached.
25. The device according to claim 22 wherein the back acoustic
cavity is defined by a conductive member which covers the entire
semiconductor substrate in which the integrated microphone is
formed.
26. The device according to claim 22 wherein the diameter and
length of the acoustic port is such as to produce a peak in the
output response in the frequency range 2.8-4.5 KHz.
Description
BACKGROUND OF THE INVENTION
This invention relates to electroacoustic transducers, such as
microphones, and in particular to a structure which is incorporated
into a semiconductor substrate.
Demand is growing for electroacoustic transducers which may be
formed as part of a semiconductor integrated circuit. These
transducers may include, for example, microphones incorporated into
the circuitry of telecommunications and audio recording equipment,
hearing aid microphones and receivers, or miniature speakers. In
the case of microphones, electrostatic device technology presently
in widespread use generally takes the form of a metalized polymeric
foil (which may be charged) supported over a metalized backplate or
stationary structure so as to form a variable capacitor responsive
to voiceband frequencies. While adequate, such devices are
relatively large, discrete components which cannot be integrated
into the semiconductor integrated circuitry with which they are
used.
Recently, such an integrated microphone structure and a method of
manufacture were proposed. (See U.S. Patent application of I. J.
Busch-Vishniac et al., Ser. No. 469,410, filed Feb. 24, 1983 and
assigned to Bell Telephone Laboratories, which is incorporated by
reference herein.) Briefly, the microphone included a tensioned
membrane formed from a thinned portion of a thicker semiconductor
substrate. The membrane had an area and thickness such that it
vibrated in response to incident sound waves. A pair of electrodes
formed a capacitor, with one of the electrodes vibrating with the
membrane to vary the capacitance when a biasing voltage was applied
and produce an electrical equivalent to the acoustic signal. It has
also been suggested that an integrated capacitive microphone can
include an insulating layer with fixed charge for providing a
built-in diaphragm bias for the device. (See U.S. Patent
application of W. S. Lindenberger, T. L. Poteat, and J. E. West,
Case 2-2-24, filed on an even date herewith and assigned to Bell
Telephone Laboratories.)
While such structures offer considerable promise for the
replacement of the distinct microphones now in use, several
problems and considerations remain in the commercial realization of
an integrated microphone. Foremost, it is desirable to make the
area of the vibrating element as small as possible to reduce cost.
However, a small area tends to cause a drop in sonic force on the
diaphragm element, thereby lowering the sensitivity of the device.
Further, smaller area diaphragms produce a smaller device
capacitance which in turn tends to increase the noise associated
with on-chip circuitry coupled to the device and also tends to
further decrease the integrated microphone sensitivity through
capacitance divider action. In order to alleviate such effects of
reduced area (i.e., reduced signal-to-noise ratio), it is desirable
to reduce the stiffness of the diaphragm.
The above-noted effects of reduced diaphragm area may also be
compensated for by a reduction in the thickness of the air gap
between capacitor electrodes. We have found, however, that for air
gaps below approximately 1.5 .mu.m with other dimensions optimized
for certain telephone applications, the electrical output frequency
response of a microphone with a tensioned diaphragm had a tendency
to fall at an unacceptable rate with frequency when utilizing
acoustic venting means, common in commercially available devices,
comprising 4-20 holes around the periphery of the stationary
electrode and backplate. That is, the devices would be overdamped,
even at the critical 300-3,500 Hz portion of the audioband which is
transmitted in telephone equipment. Specifically, with an air gap
of 0.25 .mu.m which yielded the optimum signal-to-noise ratio, the
response fell more than 20 dB across the telephone frequency band
indicating severe overdamping. Sensitivity levels were also
inoperably low. It is, therefore, also desirable to provide some
acoustic venting means which will permit reduced area and produce
an acceptable output signal at telephone band frequencies or other
frequency bands of interest.
A reduction in the air gap thickness will also have another
beneficial effect, which is to reduce the external dc voltage level
needed to bias the diaphragm. This would provide an alternative to
the requirement of a built-in diaphragm bias as suggested in the
application of Lindenberger, previously cited.
In addition, a silicon integrated microphone will generally have a
nonrising output response as a function of frequency in the audio
bandwidth. In some applications, it may be desirable to tailor the
response to provide a peak at a certain frequency by means of an
appropriately shaped acoustic port and coupling cavity. Further,
the microphone chip may, under certain circumstances, be subject to
high electromagnetic interference (EMI), and so some shielding
means may be needed. In the design of an integrated microphone,
therefore, it is desirable to provide these functions with a
minimum number of piece parts.
It is therefore an object of the invention to provide an integrated
electroacoustic transducer with a small diaphragm area which still
provides an acceptable frequency response and signal-to-noise
ratio. It is a further object of the invention to provide an
electroacoustic transducer which can be operated at a low dc bias.
It is a still further object of the invention to provide acoustical
interconnection and tuning means, and EMI shielding means, for an
integrated electroacoustic transducer.
SUMMARY OF THE INVENTION
These and other objects are achieved in accordance with the
invention which is an electroacoustic transducer formed within a
semiconductor substrate. The transducer comprises a diaphragm which
vibrates in response to an input signal at audio and/or ultrasonic
frequencies, and a pair of electrodes placed with respect to said
diaphragm so that the electric field between the electrodes varies
in relation to the vibrating diaphragm to permit conversion between
electrical and acoustic signals. In accordance with one aspect of
the invention, the diaphragm is essentially nontensioned and has a
mechanical bending moment stiffness which allows it to vibrate at
audioband and/or ultrasonic frequencies.
In accordance with a further aspect of the invention, the
transducer comprises an air gap formed between the electrodes and
acoustic venting means comprising a continuous array of small holes
extending through one of the electrodes to the air gap, where the
array covers essentially the entire area of the electrode
co-extensive with the diaphragm.
In accordance with a further aspect of the invention, a front
acoustic cavity is defined by a portion of a semiconductor
substrate adjacent to the diaphragm, an air gap is formed between
the electrodes, and a back acoustic cavity is formed adjacent to
one of the electrodes and defined by a member which also shields
the transducer from electromagnetic interference. Acoustic venting
means is provided by an array of holes connecting the air gap to
the back acoustic cavity. An acoustic port is defined by an element
adjacent to the front acoustic cavity, with the size of the port
being such as to produce a desired frequency response
characteristic for the transducer.
BRIEF DESCRIPTION OF THE DRAWING
These and other features of the invention are delineated in detail
in the following description. In the drawing:
FIG. 1 is a cross-sectional view of a microphone in accordance with
one embodiment of the invention;
FIG. 2 is a top view of a portion of the device illustrated in FIG.
1;
FIG. 3 is an illustration based on computer modeling of typical
frequency response curves predicted for microphones fabricated in
accordance with the invention as compared with predictions of a
prior art type device where the reference level is the 400 Hz
response level of curve B;
FIG. 4 is a cross-sectional view of a microphone in accordance with
a further embodiment of the invention;
FIG. 5 is a cross-sectional view of a microphone in accordance with
a still further embodiment of the invention; and
FIG. 6 is a cross-sectional view of a microphone in accordance with
a still further embodiment of the invention.
It will be appreciated that for purposes of illustration, these
figures are not necessarily drawn to scale.
DETAILED DESCRIPTION
Many of the basic features of the invention will be described with
reference to the particular embodiment illustrated in FIG. 1. It
will be appreciated that the Figure shows only a small portion of a
semiconductor substrate which in this example includes a great many
other identical, integrated, electronic transducer devices which
are separated along saw lines, 40, following batch processing and
testing.
The semiconductor substrate, 10, is a standard p-type silicon wafer
having a thickness of approximately 300-600 .mu.m (12-24 mils) and
a <100> orientation. It will be appreciated that n-type
wafers and other crystal orientations may be employed. A surface
layer, 11, of p+ type is formed over at least portions of the
substrate, for example, by an implantation of boron impurities to a
depth of approximately 0.2 .mu.m. The impurity concentration of
this region is typically 5.times.10.sup.+19 /cm.sup.3.
Formed over the semiconductor substrate is a monocrystalline
silicon epitaxial layer, 12. An appropriate area of the
semiconductor substrate is etched to form a front acoustic cavity,
13, so that the portion, 14, of the epitaxial layer over the cavity
forms the vibrating diaphragm element of the microphone. The
characteristics of the diaphragm are discussed in more detail
below.
Formed on the surface of the diaphragm is a metal layer, 15, such
as Ti-Au, which comprises one electrode of a capacitor. This layer
is typically 0.1-0.5 .mu.m thick. Formed over the epitaxial layer
is a spacer layer, 16, which is typically polycrystalline silicon
and can be formed by chemical vapor deposition. The thickness of
the layer in this example is 0.60 .mu.m and will generally be in
the range 0.1 .mu.m-4.0 .mu.m. A backplate layer, 17, is formed on
the spacer layer with a portion (hereinafter the backplate) over
the area of the diaphragm to establish an air gap, 18. The
backplate may comprise a layer of BN or Si.sub.3 N.sub.4 but could
be any insulating layer or layers. The thickness of the layer is
approximately 12 .mu.m and will generally be in the range 6
.mu.m-30 .mu.m. A metal or other conducting layer, 19, is formed on
the surface of the backplate facing the air gap and comprises the
second electrode of the capacitor. It can be the same material and
thickness as the first electrode, 15.
Formed through the backplate and electrode, 19, to the air gap is a
continuous array of holes, 20, for acoustic venting. This feature
will be discussed in more detail below.
The electronics for driving the device may be fabricated in the
adjacent area of the semiconductor substrate designated 21, and
electrical contact to the electrodes may be provided through via
holes, 22 and 23. Contact to the outside may be provided through
via holes, 23 and 24, and contact pads, 25 and 26, formed on the
surface of the backplate layer. If desired, additional via holes
(not shown) could be provided from the electronics to the outside
to establish separate dc supply, signal and ground leads.
In operation, a dc bias is applied to electrodes, 15 and 19, and an
acoustic signal is made incident on diaphragm, 14, through the
front acoustic cavity. The signal causes the diaphragm to vibrate,
thus varying the spacing between the electrodes and the capacitance
of the capacitor. This change in capacitance can be detected as a
change in voltage across some load element, integrated into area,
21, such as a second capacitance and parallel resistance (not
shown), and an electrical equivalent to the acoustic signal is
produced. The array of holes, 20, permits escape of air in the gap,
18, so that air stiffness in the gap is not a significant factor in
the diaphragm motion. Desirably, the amplitude of the output signal
as a function of frequency will be as shown in curve B in FIG. 3,
where the signal (at constant sound pressure amplitude) is
essentially flat or falls no more than 3 dB over the portion of the
audioband transmitted in telephone applications (0.3-3.5 KHz). In
accordance with a feature of the invention, the vibrating diaphragm
is essentially nontensioned so that the stiffness of the diaphragm
is dominated by the mechanical bending moment. This can be
contrasted with previous designs for integrated microphones where
the diaphragm was a thinned portion of the semiconductor substrate
and had a stiffness which was dominated by tensile stress resulting
from a heavy boron impurity concentration.
The advantage of using the nontensioned diaphragm results from the
fact that the sensitivity (e) of the microphone depends upon the
following parameters: ##EQU1## where V is the dc bias voltage
across the electrodes, A is the area of the diaphragm, k.sub.m is
the effective mechanical lumped parameter (piston-like) stiffness
of the diaphragm associated with the diaphragm's linearly averaged
deflection, k.sub.a is the stiffness of the rear acoustic cavity
(shown, for example, in FIG. 4), and h.sub.e is the distance
between capacitor electrodes, linearly averaged over A, when the
bias is applied to the electrodes. Thus, for the same sensitivity,
the area, A, of the diaphragm may be reduced by decreasing the
stiffness k.sub.m of the diaphragm (k.sub.a contributes less than
1/3 of the total stiffness acting on the diaphragm). We have found
that when the stiffness is due primarily to tensile stress, a
practical lower limit exists for reducing the membrane stiffness.
In the present invention, however, k.sub.m is significantly lowered
in order that the diaphragm area may be reduced by allowing the
mechanical bending moment of the diaphragm to be the dominant
mechanical stiffness component. Further, when the stiffness is due
to the mechanical bending moment, the thickness of the diaphragm
can be significantly increased over a tensioned membrane and still
produce a significantly lower stiffness and diaphragm area.
It will be appreciated that effective stiffness k.sub.m, as used in
this application, is the stiffness of a piston-like model where the
deflection of the piston is equal to the diaphragm deflection
averaged over the area of the diaphragm. For a nontensioned
diaphragm, this stiffness is given approximately by the
relationship: ##EQU2## where D.sub.p is the flexural rigidity of
the diaphragm.
In this example, the diaphragm thickness is 3.0 .mu.m, the
diaphragm is circular with a radius of 700 .mu.m, k.sub.m and
k.sub.a are approximately 625 and 175 N/m respectively. The biasing
voltage across the electrodes is 1.3 volts. The resulting
sensitivity is approximately -49 dB relative to 1 V/Pa at 400 Hz
(excluding any signal voltage amplifiers that may be integrated on
the chip microphone). For most nontensioned diaphragm applications,
it is expected that the diaphragm thickness will range from 1
.mu.m-5 .mu.m, and area will range from 0.4 mm.sup.2 -5 mm.sup.2 to
achieve proper sensitivity. Stiffness, k.sub.m, of the nontensioned
diaphragm is desirably less than 5500 N/m.
While in this example the diaphragm was formed from a single
crystal epitaxial layer, 12, other types of nontensioned diaphragms
might be employed. However, use of an epitaxial layer provides many
advantages in terms of processing. For example, diaphragm thickness
can be closely controlled by growing the layer over the boron-doped
surface region, 11, and then utilizing an etchant which removes the
semiconductor substrate but stops at the region, 11. The portion of
region, 11, under the diaphragm can then be removed by applying an
appropriate etchant such as KOH and H.sub.2 O for a predetermined
period of time. As shown in FIG. 1, if desired, the etchant may be
allowed to penetrate the epitaxial layer-substrate interface to
achieve a desired diaphragm thickness less than the original
epitaxial layer thickness. Close control of a nontensioned
diaphragm's thickness, d, is important since k.sub.m
.alpha.d.sup.+3. This step may also be desirable to remove boron
impurities which may have diffused into the epitaxial layer from
the substrate and added tension to the layer. A further advantage
is the fact that the epitaxial layer can be anisotropically etched
to provide via holes (22, 23, 24) or other useful features.
It will be appreciated that, in this example, the epitaxial layer
is essentially free of impurities so that there is essentially no
tension component contributing to the stiffness of the diaphragm.
However, it is contemplated that layers may be fabricated with some
tension component, as alluded to above, and still produce
advantageous results. The invention is therefore directed to
"essentially nontensioned" diaphragms which are intended to include
those having a bending stiffness contributing at least 2/3 of the
total stiffness of the diaphragm. It is further contemplated that
impurities may be intentionally introduced into the epitaxial layer
to satisfy certain needs. For example, the introduction of
phosphorus impurities into the layer could provide compression in
the layer to counteract any tension that might be produced by the
presence of boron impurities.
In accordance with another aspect of the invention, the acoustic
venting means comprises a continuous array of small holes, 20,
extending through the backplate and electrode, 19, which array
extends over essentially the entire area of the backplate and
electrode portion co-extensive with the diaphragm. Such an array
permits a narrow distance between electrodes, 15 and 19, without
overdamping, thereby permitting a reduction in the applied dc bias
and in the area of the diaphragm.
FIG. 2 is a top view of some of the holes in the array, which are
greatly enlarged for illustrative purposes. It will be appreciated
that essentially the entire area of the backplate is covered by
these holes and consequently the array is considered to be
continuous. The dynamic magnification factor, D, is defined in this
application as the amount, in dB, that the frequency response rises
in traversing the band from 400 Hz to the upper bound frequency of
interest. For telephone applications where the upper bound
frequency is 3500 Hz, it is desirable that D be greater than 31 3.0
dB. FIG. 3 shows a generally desirable, calculated, output
characteristic (curve B) for telephony, which is achieved in this
example in accordance with the invention. Returning to FIG. 2, such
a characteristic can be achieved by a radius, r, of the holes and a
center-to-center spacing of the holes, 2.xi., which satisfies the
relationship: ##EQU3## where D is the minimum desirable
magnification factor, .omega. is the highest acoustic input
frequency which will be transmitted expressed in radians (here,
2.pi..times.3.5 KHz), .omega..sub.n is the natural radian frequency
of vibration of the diaphragm (.sqroot.K/M), K is the combined
stiffness, k.sub.m +k.sub.a, of the diaphragm and back acoustic
cavity, M is the effective lumped parameter mass of the diaphragm
compatible with the prior definition of k.sub.m (which, for a
nontensioned diaphragm is approximately 9/5 times the actual mass),
R.sub.c is the critical damping level (2M.omega..sub.n), and
R.sub.a is the actual acoustic damping level of the air film in the
air gap between electrodes. The acoustic damping is determined from
the following: ##EQU4## where N is the number of holes, X is the
local "radius-of-action" associated with each hole (i.e., the
radius of the approximate circular area of air which will be vented
through each hole (see FIG. 2), .eta. is the coefficient of dynamic
viscosity of air, and h.sub.e was defined following equation (1). B
is given by the equation: ##EQU5## It will be further appreciated
that the geometry of the array yields the following relationships:
##EQU6## where .lambda. is determined by the hole configuration, A
is the area of the backplate and electrode portion co-extensive
with the diaphragm area, N is the total number of holes, and .nu.
is the fraction of the backplate or electrode, 19, which is not
consumed by the holes. For the pattern in this example, where the
center of each hole lies at the corner of an equilateral triangle,
.lambda.=1.05. For other patterns of holes, the backplate area (A)
can easily be expressed as a unique function of .xi. times N, thus
determining X and .lambda. from equations 6 and 7 above. For a
square array of holes, for example, .lambda. is 1.13. Also, if
noncircular holes are used, the above relationships may still be
used for first order calculations if the radius, r, is equated to
##EQU7## where A.sub.h is the area of the hole.
Thus, given the desired magnification factor D, the area, A, and
mass of the diaphragm, M, the average distance between the
electrodes when the bias is supplied, h.sub.e, and the combined
stiffness of the diaphragm and back acoustic cavity, K, the above
equations can be solved to give combinations of hole radius and
center-to-center spacing (r, 2.xi.) or hole radius and number (r,
N) which can be utilized for acoustic venting in accordance with
the invention. The preferred combination is that which consumes the
minimum amount of electrode area. (For a detailed discussion of the
derivation of relationships governing acoustic impedance of the air
gap in electrostatic transducers due to holes in one of the
electrodes, see Skvor, "On the Acoustical Resistance due to Viscous
Losses in the Air Gap of Electrostatic Transducers," Acustica, Vol.
19, pp. 259-299 (1967-68), which is incorporated by reference
herein.)
In this example, D is -0.8 dB, h.sub.e is 0.56 .mu.m, N is
approximately 2000, r is 9.8 .mu.m and 2.xi. is 29.8 .mu.m for a
diaphragm with area of 1.54.times.10.sup.-6 m.sup.2, and effective
mass of 1.93.times.10.sup.-8 kgm. This leaves a total area not
consumed by holes of 61% (.nu.) of the backplate or electrode area.
The capacitance is still sufficient, however, to produce a
sufficiently high output signal as specified by the sensitivity
previously given.
Of course, the above parameters may be varied according to specific
needs. It is recommended, however, that there be a minimum of at
least 50 holes per square millimeter to avoid overdamping in the
output signal and that each hole have a diameter of less than 100
.mu.m to allow sufficient diaphragm capacitance (at least 1 pF) for
operation of the transducer. For the sake of comparison, curve A of
FIG. 3 shows the calculated frequency response for a hypothetical
microphone having the same dimensions as described in the example
shown by curve B, but not including the venting means of the
invention. As compared with the microphone of curve B which
includes 2000 holes (1300 holes/mm.sup.2) each having a diameter of
19.6 .mu.m, the microphone of curve A includes only 20 holes (13
holes/mm.sup.2) each having a diameter of 196 .mu.m. Both designs
have the same electrode area not consumed by holes (61%) so that
the capacitances are equal and at least the potential
signal-to-noise ratio is the same for both. Nevertheless, curve A
shows a severely overdamped frequency response.
It should be appreciated that the average spacing between
electrodes (h.sub.e) when a bias is supplied should not vary too
much from the air gap (h) with no bias applied if the system is to
remain stable. It is recommended, therefore, that h.sub.e be 4-10%
less than h. In this example, h.sub.e =0.94 h.
As noted previously, a reduced air gap, which is possible with the
venting means of the invention, should also permit a reduction in
the external dc bias needed for operation. For a gap, h, between
electrodes of less than 3.0 .mu.m, it is expected that the
microphone can be operated at less than 5 volts supplied to the
capacitor electrodes.
It should also be appreciated that, although the air hole array is
described with use of a nontensioned diaphragm, the hole array as
described heretofore may also be used with tensioned diaphragms
such as that shown in application of Busch-Vishniac, cited
previously. In such cases, the area of the diaphragm may be larger
than that for the nontensioned diaphragm, but would still be,
advantageously, less than 8 mm.sup.2.
It should be further appreciated that while the above relations
allow one to specify a uniform air hole array (that is, a constant
hole size and pattern), a somewhat nonuniform pattern that might
possibly be desired may be designed by applying equation (4)
piecewise across the backplate and electrode, and summing over N
(holes).
The air gap, 18, electrode, 19, backplate layer, 17, and the air
hole array, 20, may be conveniently formed by known deposition and
photolithography steps. For example, layer 16, which may comprise
polycrystalline silicon, can be deposited by chemical vapor
deposition and the area which will comprise the air gap is then
defined by selectively etching the layer. An etch-stop material,
27, such as BN or Si.sub.3 N.sub.4 can be formed around the walls
of the hole. The hole is then filled with a material such as
polycrystalline silicon or SiO.sub.2 and planarized. The electrode,
19, may then be formed by a selective deposition leaving the
desired hole array therein. The backplate layer, 17, which may
comprise BN or Si.sub.3 N.sub.4 or a combination of like materials,
is then deposited and the corresponding hole array formed therein
by standard photolithography. The filler material can then be
removed from the air cavity by applying another etchant through the
hole array. Of course, during these various etching operations, the
via holes, 22, 23 and 24, needed for interconnection can also be
formed.
It will be appreciated that while the above example employed a
circular diaphragm and backplate, the principles of the invention
may be applied to any shaped diaphragm and backplate.
In accordance with a further aspect of the invention, various
acoustical interconnection means and EMI shielding means may be
incorporated into the basic microphone structure previously
described. For example, FIG. 4 illustrates the formation of a back
acoustic cavity, 30, adjacent to the air hole array, 20. This
cavity is formed within a carrier substrate, 31, against which the
silicon microphone structure is placed. This substrate can be a
printed wiring board or other carrier substrate to which electronic
components are usually attached.
Coupled to the front cavity, 13, is an acoustic port, 32, which is
formed from an element, 33, which is typically a plastic closure.
The acoustic port adds a degree-of-freedom to the microphone system
and adds a peak to the frequency response of the device to serve
various needs. Thus, in the present example, the response shown as
curve C in FIG. 3, where the peak is placed near the upper end of
the telephone band, is obtained from the microphone characterized
by curve B simply by adding an acoustic port having a diameter of
150 .mu.m and a length of 1600 .mu.m. In this example, the cavity,
13, has a volume of 0.94 mm.sup.3. Long holes with narrow diameters
yielding high acoustic mass are generally needed in this silicon
microphone application due to the large stiffness of the small,
front acoustic cavity. In general, diameters of 100-180 .mu.m and
lengths of 600-2000 .mu.m are expected to be useful for producing
peaks where desired. For telephone applications, it is desirable to
form the peak within the frequency range 2.8-4.5 KHz. Although not
illustrated in these figures, all members forming acoustic ports or
cavities are acoustically sealed by standard means such as with
adhesives or by clamping.
If shaping of the frequency response, as shown in curve C, is not
needed, the embodiment shown in FIG. 5 might be utilized. Here, the
cavity, 30, formed in carrier substrate, 31, acts as an extension
of front cavity, 13, and the acoustic signal is made incident on
the diaphragm through the hole array, 20. In this embodiment, in
fact, the cavity extension, 30, may be eliminated so that the
acoustic cavity is formd entirely within the semiconductor
substrate. In any event, no extra parts are needed to form the
acoustic interconnections. A further advantage is that electrical
contact can be made to the microphone by wire bonds, 34 and 35,
from the carrier substrate, 31 to the top of the backplate
layer.
In the embodiment illustrated in FIG. 6, sound is again incident on
the diaphragm through an acoustic port, 32, coupled to the front
cavity, 13. Here, however, the sound port is formed in the carrier
substrate, 31, so that the microphone is again mounted with the
backplate side-up permitting wire bonding. Additionally, an
enclosure member, 36, is provided surrounding the entire
semiconductor microphone. This member can be made of conductive or
conductively plated plastic or metal so as to provide a shield for
the device against electromagnetic interference. At the same time,
the member forms a back acoustic cavity, 30, for the microphone.
The member can be grounded, for example, by bonding to grounded
pad, 37, formed on the carrier substrate. Thus, EMI shielding is
provided with a minimum of piece-parts.
It should be appreciated that although the acoustical
interconnection means and EMI shielding means are described with
use of a nontensioned diaphragm, such may also be used with
tensioned membranes as for example shown in application of
Busch-Vishniac, cited previously.
It will be appreciated that the inventive features discussed herein
could also apply to a pressure gradient type microphone where sound
is allowed to strike both sides of the diaphragm, thus effecting a
noise-canceling and directional response. To produce such a device
in FIGS. 4 and 5, a secondary sound port would simply be placed in
the carrier substrate, 31, while in the embodiment illustrated in
FIG. 6 a small secondary port would be placed through enclosure
member, 36. In any case, the second side of the diaphragm is
accessed.
It will also be appreciated that although the above discussion has
focused on the microphone, the principles of the invention may also
be applicable to other types of electroacoustic transducers
utilizing a capacitor whose capacitance varies in accordance with a
vibrating diaphragm, whether an acoustic signal is converted to an
electrical signal or vice-versa. For example, a loudspeaker or
hearing aid receiver might be fabricated by applying a varying
electrical signal to the capacitor electrodes (15 and 19) which
causes vibration of the diaphragm (14) due to the varying
deflection of the electrode (15) attached thereto. An acoustic
output signal would therefore be produced. Thus, whichever way the
energy conversion is taking place, the electric field between the
electrodes varies in relationship with the vibrating diaphragm to
permit conversion between electrical and acoustic signals.
It will also be realized that the invention is not limited to
telephone band frequencies (0.3-3.5 KHz) but can be used in the
full audio bandwidth (0.02-20 KHz). In fact, this silicon
transducer invention can find application in the ultrasonic band
(20-1000 KHz).
Various additional modifications will become apparent to those
skilled in the art. All such variations which basically rely on the
teachings through which the invention has advanced the art are
properly considered within the spirit and scope of the
invention.
* * * * *